The Growing Concern of Soil Heavy Metal Contamination
Soil heavy metal (HM) contamination is becoming an alarming global problem, largely fueled by industrial expansion and various human activities. As industries grow, so too does the likelihood of HMs finding their way into our soils. Key contributors include mining, pesticide application, battery manufacturing, plating processes, textile production, petrochemical processing, and metallurgy. Several studies have highlighted that these anthropogenic activities lead to an increase in HM levels that far exceed natural occurrences (Zhou et al., 2020; Qasem et al., 2021). The ramifications of exposure to these toxic elements can be profound, raising significant health concerns as HMs can lead to various toxicological effects, some of which classify certain metals as confirmed or potential carcinogens (Khan et al., 2015; Briffa et al., 2020). Disturbingly, it is estimated that about 20% of arable land in China alone faces HM pollution (Sun et al., 2019), highlighting the widespread nature of this issue.
Advances in Soil Remediation Techniques
The increasing awareness surrounding HM pollution has sparked a robust search for effective materials and methodologies to mitigate these environmental contaminants. Soil remediation, aimed at reducing the hazardous effects of HMs, is primarily conducted through two methods: ex-situ and in-situ remediation. Ex-situ remediation entails the removal of contaminated soil for treatment off-site, whereas in-situ remediation occurs at the contamination site without the removal of the soil. In-situ strategies can involve washing, phytoextraction, and various mechanisms for immobilizing the HMs within the soil itself (Lombi et al., 2001; Li et al., 2016; Park and Son, 2017; Peng et al., 2018; Kheir et al., 2021). The geochemical processes at play in in-situ immobilization include sorption, ion exchange, electrostatic attraction, precipitation, and complexation, collectively working to minimize the adverse impacts of HMs on environmental and human health (Shaheen et al., 2015; Lahori et al., 2017b; Hamid et al., 2020c; Arabi et al., 2021).
The Role of Biochar in Remediation
One promising material emerging in the realm of HM remediation is biochar (BC). This carbon-rich material, produced via the pyrolysis of biowaste feedstocks under oxygen-limited conditions, has gained recognition for its potential to not only capture carbon emissions but also to remediate contaminated soils (Lehmann et al., 2011; Chagas et al., 2022). BC features a highly porous structure paired with chemically reactive surface groups, enabling it to effectively immobilize HMs through complexation, adsorption, and ion exchange processes. Studies indicate that biochar can significantly decrease the availability of HMs in soil, transforming them into less accessible forms, thus preventing their uptake into the food chain and their percolation into groundwater (Hamid et al., 2019; Gondek et al., 2023).
Moreover, the introduction of BC to soil enhances various properties such as soil health and crop productivity. The presence of biochar fosters the formation of soil aggregates, beneficial for microbial habitats, while also contributing to chemical changes such as increased soil pH and enhanced phosphorus availability (Bolan et al., 2023; Figueiredo et al., 2019). However, it’s crucial to remain mindful that the effectiveness of biochar can vary based on its specific properties and the characteristics of the soil it is applied to. Additionally, depending on the feedstock used, biochar can unintentionally introduce HMs and other toxic contaminants back into the soil, imposed by poor feedstock selection (Bolan et al., 2024).
Zeolites: Nature’s Ally in Soil Remediation
Complementing biochar in the battle against soil contamination are zeolites. These naturally occurring hydrated aluminosilicate minerals possess a unique structure that allows for reversible hydration and cation exchange without significant alteration of their overall form (Ming and Mumpton, 1989). Zeolites are being actively researched for their applications in soil remediation, where their inherent properties of adsorption and cation exchange come into play (Ming and Boettinger, 2001; Chang et al., 2019). With an abundance of natural zeolite reserves and the capability to synthesize these minerals, their promise as effective remediators has gained traction (Velarde et al., 2023; Senila and Cadar, 2024).
The interaction of zeolites with soil extends beyond mere remediation; they significantly influence soil’s physical, chemical, and biological properties. For instance, zeolites can modify soil porosity and connectivity, fostering a more favorable environment for soil biota while simultaneously inhibiting the proliferation of soil-dwelling plant pathogens (Javaid et al., 2024). This adaptability makes zeolites invaluable not only for remediating HM but also for enhancing overall soil health, especially under conditions of salt or water stress in plants (Bybordi, 2016).
The Synergistic Effect of Biochar and Zeolites
Research indicates that the combined application of biochar and zeolites can lead to superior outcomes in the immobilization of HMs (Feng et al., 2022; Gondek et al., 2023). The interaction of these two materials is not merely additive; BC primarily aids with HM immobilization through its liming effect and high phosphorus content, while zeolites enhance soil cation exchange capacity and encourage soil alkalinization (Boostani et al., 2018). Furthermore, recent studies are exploring the enhancement of BC and ZE combinations with various additive materials (BC + ZE + A), such as lime, phosphates, clay minerals, and organic compounds, to boost remediation effectiveness (Lahori et al., 2019; Lahori et al., 2020a; Hamid et al., 2020b; Li et al., 2021).
Evidence shows that this composite approach can significantly decrease the bioavailability of HMs in soil, ultimately preventing their accumulation in edible parts of plants (Lahori et al., 2017a; Bashir et al., 2018c; Gu et al., 2018). Understanding the underlying processes of biochar-HM interactions clarifies the mechanisms of HM stabilization in amended soils. For instance, the immobilization of lead ions can occur through several processes such as electrostatic complexation with available cations in biochar, co-precipitation, surface complexation, and more, further underlining the multifaceted interaction of these innovative materials (Ahmad et al., 2014; Netherway et al., 2019).
Exploring the Future of Soil Remediation Research
Despite the advancements in the field, comprehensive understandings of the concurrent use of BC and ZE for remediating specific HMs like Cd, Cu, Pb, and Zn remain limited. To address this gap, ongoing research, including meta-analyses, aims to investigate the effectiveness of these materials, both individually and in combination, for reducing HM availability in soils. The study further proposes to explore how additional additives can amplify the performance of BC and ZE combinations. Subgroup analyses will play a crucial role in determining the variables that impact the remediation of cadmium, thereby extending our existing knowledge and opening avenues for future studies in this vital area.
Through these investigations, scientists hope to enhance soil health, improve agricultural productivity, and ultimately protect human health and the environment from the damaging effects of heavy metal contamination. It is an intersection of science and responsibility, where understanding and innovation could pave the way for a cleaner, more sustainable future for our soils.